How is it best to distribute a complex unity project to other devs as asingle dll

I have a complex Unity project that I would like to distribute to a couple of other devs. They need to be able to write some new classes that subclass a particular class in the project.

However it's a requirement that the code content of the main project itself, preferably including the class they need to subclass, is not accessible to the devs. [EDIT: It's only the source code they can't access. They can still use the class, and will have documentation.]

Ideally the project I distribute would include one dll that contains the all of the classes from the original project.

The approach I have taken is as follows (it may be overly simplistic - I've not worked with .NET outside a Unity context)

1) Take the original project and get the Assembly-CSharp.dll that it has compiled from the Library/ScriptAssemblies folder

2) Create a new project, and place the dll in the Assets folder and rename it.

Note:Not renaming it causes the following error - I'm assuming the name is reserved

ApplicationException: Unable to find a suitable compiler UnityEditor.Scripting.ScriptCompilers.CreateCompilerInstance (MonoIsland island, Boolean buildingForEditor, BuildTarget targetPlatform) (at C:/BuildAgent/work/812c4f5049264fad/Editor/Mono/Scripting/ScriptCompilers.cs:99)

T$$anonymous$$s works and satisfies the requirement of having one dll that i could distribute.

The problem I have occurs when I add new scripts that try to subclass a class that is wrapped in the dll.

So for instance if the dll packages a class such as

  namespace MyNameSpace {
     
     public class  Fruit: IFruit  {
 
     }
     ..
     ..
    
  }

and I create a new script called Banana that attempts to subclass Fruit

 namespace MyNameSpace
 {
 
     public class Banana : Fruit, IFruit {
 
     }
     ..
     ..
 
 }

T$$anonymous$$s causes a runtime exception TypeLoadException: Could not load type 'MyNameSpace.Banana' from assembly 'Assembly-CSharp, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null'.

I haven't so far been able to find out why t$$anonymous$$s is. Is it related to changing the name of the dll? Or is there somet$$anonymous$$ng fundamental about the Assembly-CSharp.dll that i took from the original project that mean it can't be used in t$$anonymous$$s way? As I say I don't have a background in .NET, so I realise what I'm doing maybe somewhat naiive.

Also, I have experimented with compiling individual scripts, and groups of scripts, into dlls, and extending classes embedded i those dlls without any problems. I've used Assembly Builder ( http://studio.foriero.com/products/assembly-builder/assembly-builder.php ) to do t$$anonymous$$s and it works fine for compiling a limited number of scripts (w$$anonymous$$ch it is what it's built for). It seems to have problems with large numbers of classes. So if I try to build my entire project like that it will crash. I could also look at compiling the project into a number of dlls, however I have some significant refactoring I'd have to do, and t$$anonymous$$s is time sensitive.

I'm also open to suggestions of completely different ways of doing t$$anonymous$$s, it may even be that I'm missing somet$$anonymous$$ng really obvious?

Anyway, I appreciate any help I can get!